Small Scale Dredging
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SMALL SCALE DREDGING AN INTRODUCTION IT’S IN OUR NATURE Small-scale dredging TABLE OF CONTENTS 1. INTRODUCTION: SMALL-SCALE DREDGING………………………………………………………………….. 2 2. CRITERIA FOR SELECTING DREDGING TECHNIQUE……………………………………………………….. 3 3. DREDGING TECHNIQUES……………………………………………………………………………………………… 5 4. TRANSPORT OF DREDGED MATERIALS……………………………………………………………………….. 17 5. STORAGE OF DREDGED MATERIALS……………………………………………………………………………. 21 “Small-scale dredging” Conver 2014 1 / 22 Small-scale dredging 1. INTRODUCTION: SMALL-SCALE DREDGING The word dredging probably originates from the old English word “dragan”, or to draw. Dredged materials normally involve a layer of soft matter, which is formed when plants, waste, soil material and leaves cling to the bottom of waterways. In time, this can have an impact on shipping traffic or the capacity of the waterway in question. Dredging is carried out for a variety of reasons. However, in most cases, dredging is done for maintenance purposes and to ensure sufficient water flow. Small-scale dredging projects generally involve drainage channels and modified rivers in areas with artificial (pumped) drainage and smaller urban and suburban waterways, which are not used for shipping activities. Photos: Reduced water flow 2 / 22 Small-scale dredging 2. CRITERIA FOR SELECTING DREDGING TECHNIQUE A variety of dredging techniques have been developed over the years. When preparing for dredging activities, a decision must be made of which dredging technique is best suited to the activities in question. When doing so, the following factors must be taken into consideration: - composition of dredged materials - type and level of pollution - specific circumstances - size of project - acceptable opacification and spillage - required accuracy - side-effects - ecological considerations Composition of dredged materials The choice of dredging technique is also determined by the physical composition of the materials that must be dredged. For instance, different techniques must be used when dredging soft silt than when dredging sand or clay. Type and level of pollution Another factor which determines the choice of dredging technique is the presence of gross solids, like household and construction waste. Gross solids on the water bottom can reduce the effectiveness of certain dredging techniques. The level of pollution on the water bottom is generally not a decisive factor when deciding which dredging technique is going to be used. However, special dredging equipment has been developed to, for example, reduce unwanted dispersion and opacification during dredging activities. This equipment can be damaged by gross solids on the water bottom and cannot be used in all circumstances. The quality of (or level of pollution in) dredged materials determines whether materials can be placed at the water's edge or needs to be transported. Specific circumstances The choice of dredging technique is primarily determined by the accessibility, water depth, current speed and dimensions of the waterway in question. In urban areas, physical objects like lamp posts and parked cars are major obstacles for equipment operated from the banks of the waterway. This also applies to the width of waterways or the height of bridges when floating equipment is used. Size of project It may be necessary to dredge large stretches of water bottom when performing new infrastructure work, extracting sand and maintaining waterways. Project size is often expressed as “m3 in situ”. This refers to the volume of dredged material that must be removed. In addition, the surface area across which materials are spread, thus the layer thickness, also determines the capacity and cost of the deployed dredging technique. Acceptable opacification and spillage Materials are removed under water during the dredging process. Material released by a dredging technique, which is not subsequently disposed of via the transport system, is referred to as 'spillage'. Spillage is often manifested via a loose top layer, possibly featuring chunks of sediment. Spillage also results in a loose top layer when suspended particles (opacification) are subject to sedimentation. 3 / 22 Small-scale dredging However, polluted spillage is often encountered when dredging polluted water bottoms, which means it determines the chemical quality of the water bottom and the surface water. Accuracy There are two facets to the accuracy of a dredging technique. On the one hand, it involves the accuracy with which dredging takes place and, on the other hand, how accurately these newly dredged areas match the required profile. Accuracy is a criterion that can be clearly measured and checked. In general, it is assumed that opacification and spillage can be minimized if strict accuracy- related benchmarks are implemented. Side-effects In general, it is impossible to avoid unintentional effects of dredging activities, like opacification in the water column. However, contractors can take measures to reduce such effects, which includes installing silt screens. Other unintentional effects, like destruction of tow paths and noise-related nuisance, cause techniques with such side-effects to be avoided when working in vulnerable (natural) areas. These techniques will only be used if there are no viable alternatives, although the environment must be given maximum priority at all times. Ecological considerations There is an inter-woven relationship between ecology and dredging. Legislation relating to plants and wildlife imposes additional restrictions on available methods and available periods of the year. Therefore, ecological considerations must always be taken into account when performing dredging activities. Several dredging techniques or a combination of them may be suitable to successfully complete certain projects. That is why specifications often do not prescribe one particular technique, but only identify boundary conditions or the required end result. One statement is extremely important when it comes to small-scale dredging activities: “There is no such thing as a standard dredging project. Each project requires its own unique approach.” The following chapter will examine the most commonly used dredging techniques. 4 / 22 Small-scale dredging 3. DREDGING TECHNIQUES 3.1 Hydraulic excavator This commonly used piece of equipment - the hydraulic excavator - is a discontinuous excavator, where excavation takes place using single buckets in consecutive cycles. The full bucket is brought to the surface and emptied into the means of transport (truck or barge) or placed on the bank. Photo: Excavator on pontoon. The under-water surface is excavated in a circular pattern, where the bucket is lifted by the excavator when it reaches a certain depth. The next circle will be started once one circular section has been completed, and this process will continue until the whole waterway has been excavated. The excavator will then be moved, thus allowing another zone to be excavated or dredged. The bucket must be carefully positioned after each cycle so it is not easy to perform two consecutive excavation cycles under water without an accurate and reliable system for determining the excavation position. Water depth and working area There are two ways of implementing hydraulic excavator: - work carried out from the water's edge, - work carried out from a pontoon featuring anchors or spud poles. The eventual choice will be determined by project-specific circumstances. Depending on the selected method and the size of the excavator, it is possible to achieve water depths up to 10 m or more. Material above the waterline can also be excavated if the location is accessible to the excavation bucket and the means of transport. 5 / 22 Small-scale dredging Photo: Excavator on the bank. Available excavation buckets When using excavators to perform dredging activities in areas subject to nature-related and environmental considerations (for example, land improvement operations), various types of excavation buckets can be used to reduce excavation-related effects. Digging bucket A digging bucket excavates materials using a pulling movement and is thus often implemented to remove thin layers of silt. Once the bucket is full, it is raised to the surface and emptied into the means of transport or on to the water's edge. The dredging bucket is slightly different from standard buckets, and is characterized by holes that retain silt while allowing water to escape. Photo: Digging bucket and dredging bucket 6 / 22 Small-scale dredging The visor bucket The visor bucket is a traditional excavation bucket featuring a revolving valve (visor), which can be used to close the top of the bucket. The visor is closed once the bucket is full, so dredged materials are enclosed in the bucket while being raised to the surface. This avoids or drastically minimizes dredged material mixing with the water column. This bucket can be used to excavate thin and very dense layers with low water content. Layer thickness of around 0.5 m is needed in order to collect enough material. Photo: Visor bucket and clamshell The (closed) clamshell A clamshell or grab bucket operates using a gripping movement. That is why it is used when excavating thick layers of silt and when transferring materials. A traditional clamshell is open at the top, which means excavated materials are exposed to the water column when it is being raised to the surface. The top of closed clamshells can be sealed off, so exposure between excavated materials and the water column is avoided when it is being raised